To heat the grill or to turn on the charcoal of the barbecue. To take a casserole, to mix the shallot and 1 spoon of oil. To cook 1-2 minutes to middle fire. To unite the fumetto, the tarragon, the juice of apples, the maizena, half salt and a ground of pepper. To let thicken mixing continually until it doesn't boil. To lower the flame, to simmer for 2-3 minutes and to put back. To wash the slices of fish with cold water and to dry them with the kitchen paper. Prepare abundantly, to salt with the remained salt and to brush with the advanced oil. To cook under the grill or on the barbecue for 3-4 minutes for every part. The meat of the fish has to become opaque. An instant before the fish is ready to heat, to slim flame, the sauce. To cover the slices with the sauce, to garnish and to serve.